It drives dog owners nuts. You’re out for a pleasant stroll with your canine companion, and because some asshat didn’t pick up after their own pooch, a fresh, steamy turd is sitting right there on the sidewalk. Before you have time to react, you horrifyingly watch your supposed best friend pounce upon the poop, gobbling it down in the blink of an eye.

But why? Seriously, why?

What could possibly compel a dog to eat another dog’s poop, or in some cases, its own poop? It’s a baffling behavior, because dogs tend to be averse to the stinky stuff, not shitting in their own beds, so to speak. In a word, it’s a paradoxical behavior. Many dog owners are understandably concerned when their pets do this, fearing that something’s psychologically wrong with their dogs, or that the poop might make them sick. And no one wants to be licked by a dog who just pounded down a steaming pile of crap.

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But it’s not just dog owners—scientists are also curious and concerned about this strange behavior. In an effort to learn how common shit-eating is among dogs, and to figure out why they do it, a team led by Benjamin Hart from the University of California at Davis got thousands of dog owners to complete a pair of web-based surveys.

The first survey, involving 1,552 dog owners, assessed the frequency of this behavior in dogs, while exploring potential differences among poop-eaters and non-poop-eaters. The second survey, with its 1,475 respondents, explored the reasons behind the behavior, and to determine if commercial treatment products actually stop dogs from eating their own poop (spoiler alert: they don’t). Dogs are known to eat poop from other species, but this study focused solely on the phenomenon of “coprophagy”—the act of repeatedly eating one’s own poop, or poop from members of the same species.

Results show that 16 percent of dogs are coprophagic, meaning they have eaten poop on at least six different occasions, as observed by their owners. And dogs who eat poop tend to do it often: 62 percent ate stools daily (whoa), and 38 percent weekly. But the dogs were picky. The vast majority of the poop was fresh, no more than two days old. Any older than that, and the dogs tended to avoid it. These results were published last week in the science journal Veterinary Medicine and Science.

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Looking at the data, the researchers struggled to find connections that could explain the behavior. Previous theories have suggested stress and anxiety, or dietary deficiencies, but the new results don’t entirely support these ideas. For example, no link could be found between a dog’s age, diet, breed, neuter status, or the age at which it was separated from its mother. Poo-eaters were just as easily house trained as those who avoid the habit, “suggesting a normal aversion to feces,” write the authors. The behavior was more common in multi-dog households, but that’s probably because there’s more access to shit.

But the researchers did manage to find one potential causal factor: Owners with poop-eating dogs tended to describe their canine companions as “greedy eaters,” compared to non-coprophagic dogs. More on this in just a bit.

As for the success rate of commercial treatment options (typically food additives and pills that are supposed to make dogs averse to their poop), the researchers said their effects are “close to zero, indicating the behavior is not readily changed.” So don’t waste your money, or potentially compromise your dog’s health, with these products.

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As to why dogs do it, the researchers say it may have something to do with their unwillingness to eat poop older than two days.

As already noted, coprophagy is a paradoxical behavior for dogs. They’re naturally averse to the stuff, and they avoid pooping where they live. This aversion is likely inherited from their wild wolf ancestors, who evolved the behavior to avoid fecal-borne intestinal parasites and other pathogens. Well, it just so happens that the eggs of parasites don’t typically hatch into infectious larvae for a few days. According to the new theory, the eating of fresh poop among ancient wolves may have been a way for them to “clean up” when a sick, old, or lame wolf accidentally or unwillingly plopped some poop in its own den. Thus, poop-eating is an adaptive behavior that helped canine ancestors avoid intestinal parasites.

James Serpell, a professor at the University of Pennsylvania’s veterinary school, told the Washington Post that the theory is “plausible,” but that the “greedy eaters” observation suggests dietary motivations. Studies done elsewhere have shown that free-roaming dogs in developing countries eat a lot of human feces to make up for dietary deficiencies. What’s more, today’s dogs and cats “are fed diets that are relatively rich in fats and protein, not all of which may be completely digested, making their feces potentially attractive as a second hand food source,” Serpell told the Washington Post.

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The new theory is interesting, but not completely satisfying. Coprophagy is a very difficult issue to study, and there are many plausible reasons why dogs indulge in this behavior—from wanting to grab a quick meal or out of sheer boredom. And in fact, the reasons may vary from dog to dog, and motivated by an assortment of known and unknown factors. It’s also important to point out that this study was driven by self-reported surveys; the scientists had to rely on the assessments of dog owners, and describe such subjective traits as “greedy” eating.

While an answer to this question is still forthcoming, it does appear that poop-eating is something that dogs normally do. That doesn’t mean you have to like it, nor does it mean this it’s a completely healthy behavior for dogs (in some cases, it can cause diarrhea, and even the transmission of parasites and disease). More often than not, a dog will be okay after chomping down on a deuce, but concerned owners should bring their dog to a vet in case they’re worried. As for preventing your dog from eating poop, your best bet is to keep your backyard poop-free, and keep you dog on a leash while out for a walk.